An antiskid brake for feedback control of brake torque based on a difference between wheel speed and vehicle speed of an automobile to prevent the wheels of the automobile from locking during braking, is disclosed for example in the following references:
(1) Tan and Chin: Vehicle Traction Control: Variable Structure Control Approach, Trans. of ASME Dynamic Systems, Measurement and Control, 113, 223/230 (1991), PA1 (2) Chin, William, Sidlosky, Rule and Sparschu: sliding-Mode ABS Wheel-Slip Control, Proc. of Am. Control Conf. 1/5 (1992). PA1 wherein: PA1 wherein: ##EQU3## J.sub.w, q.sup.+, q.sup.- .delta.: constants u.sub.cmd (t): brake torque target value PA1 wherein: PA1 wherein: ##EQU5## J.sub.W, v.sup.+, v.sup.-, k.sub.W : constant x.sub.W : wheel angular acceleration
In the devices disclosed in these references, a function having vehicle speed and wheel speed as arguments is set, and a wheel slip ratio between the road surface and tires is controlled to a target value by switching the brake torque depending on whether the function is positive or negative as shown by the graph (b) of FIG. 13. However, there is a delay in brake torque generation due to the delay of an actuator that generates a brake torque or due to the time required to calculate the function, and this tends to cause oscillation (known in the art as "hunting") of the brake torque or wheel slip ratio.
To suppress such an unfavorable oscillation, the characteristics of the function may be set so that the brake torque is not changed over abruptly, but is made to vary smoothly from a maximum value to a minimum value as shown by the graph (a) of FIG. 13. However, when the brake torque is varied gradually, the control error increases so the target value and actual value of the wheel slip ratio may not precisely coincide with one another.